The inventions shown and described herein relate to methods of and apparatus for testing and adjusting direct current rotating machines such as motors and generators.
One of the problems involved in testing and adjusting d.c. machines is to select the proper positions for the brushes. Many years ago a brush was shifted back and forth in an effort to find the best position. As stated in the text Electrical Machinery, by Fitzgerald and Kingsley, published by McGraw-Hill Book Co. (1952), at page 241:
"Shifting the brushes was at one time a common way of securing good commutation. The method is now obselete, however." PA1 1. It is not practical to achieve perfectly symmetrical air gap flux distributions.
The heart of a d.c. machine is the armature. All power conversion energy flows through it, and its capability to convert mechanical rotation from or to undirectional currents and magnetic fields is largely controlled by geometrical factors. This means that the coupling medium comprised of the air gap magnetic field should be marshalled into perfectly symmetrical areas through which energy is allowed to flow (the main poles) and out of other areas where the commutation process must take place (the commutation zones).
The armatures of d.c. machines may be lap or wave wound. The armatures have multiple parallel current paths, one or more paths per pole for those having simplex or multiplex windings. If the air gap magnetic fluxes are perfectly symmetrical, the parallel currents should have equal magnitudes a factor that should contribute to good commutation. Asymmetrical air gap conditions will result in unbalanced currents.
Under steady load conditions, the currents that flow from the positive to the negative pole brushes are continuous. However, internal to each brush and at the commutator-to-brush interface the currents are continuously changing. There are three current components, two of which must be minimized or ideally, completely eliminated for good commutation. These are described below:
The armature load current is divided proportionately between the commutator bar areas bridged by the brush. The shorted coils provide a low resistance bypass such that the current division between the bars shorted by the brush is controlled by the brush-to-bar contact resistance, which is proportional to contact area. Under this condition alone the rate of change of current across the brush face is uniform resulting in uniform brush contact temperatures.
Armature parallel path imbalance currents are caused by asymmetrical air gap conditions and improper positioning of the brushes. These currents are divided between the shorted coils and the brush contact areas. Since these currents must flow from one side of the armature to the other they result in increased current densities toward the leading or lagging edges of the brushes. Since high current densities cause high contact temperatures which in turn lower the brush contact resistance and further increase current densities, the current imbalances result in sparking and burning of the brush contact faces and commutator surfaces.